‘Marine A’ has a “real possibility” of overturning his conviction for “murdering” a Taliban terrorist at an appeal hearing next month, after a review of his case found the legal team who had defended him made a series of “errors”.

The Criminal Cases Review Commission (CCRC) found that former Sergeant Alexander Blackman, known as ‘Marine A’, was failed by “deficiencies in the standard of defence” as the lawyers representing him carried out “inadequate” preparations for his court martial.

Sgt Blackman was convicted of murder in 2013, but he was granted a fresh appeal last month after a new legal team discovered evidence indicating he was suffering from an undiagnosed mental disorder when he killed the terrorist.

“Had psychiatric evidence been obtained prior to trial it would have identified Mr. Blackman’s medical condition and should have caused both Mr. Blackman and his legal representatives to re-assess the incident, his level of culpability and the nature of his defence”, the CCRC report, shown to The Daily Telegraph, said.

“There is a real possibility that the Court of Appeal would consider Mr. Blackman’s conviction for murder to be unsafe on the basis that the conduct of his defence prior to trial was deficient to the extent that it led to identifiable errors which rendered the trial process unfair. In the absence of such evidence Mr. Blackman was able to explain only partially, and in an unsatisfactory way, his actions and belief,” the document adds.

During his trial in 2013, Sgt Blackman argued that he thought the wounded terrorist was dead when he fired the lethal shot at short range after a battle.

The review body found that the former marine’s lawyers had failed to carry out basic procedures. They did not take a detailed “proof of evidence” statement from Sgt Blackman, failed to set out the events leading to the death of the insurgent, and were guilty of a “failure… to take detailed instructions and give persuasive legal advice”.

“In particular, a psychiatric report should have been obtained to assess Mr. Blackman’s state of mind at the time of the incident,” the review adds.

“There are reasons to consider that Mr. Blackman’s defence preparations were inadequate…” and “…the CCRC considers that that the defence preparation fell below the standard required”, it states.